Method and apparatus for producing a purified liquid

ABSTRACT

The present invention provides generally a method and apparatus for purifying a liquid. More particularly, purification of a bulk liquid is performed by introducing a liquid stream to a purification vessel, which comprises both a distillation chamber for forming a purified vapor and an annular chamber for collecting a purified liquid that is condensed from the purified vapor. A refrigerant system provides thermodynamic efficiency in the purification method by directing waste heat generated in one part of the refrigerant system for heating duty in another part of the refrigerant system. The method and apparatus can be applied to producing purified carbon dioxide, nitrous oxide, ammonia and fluorocarbons.

FIELD OF THE INVENTION

[0001] The present invention relates to a method and apparatus forproducing a purified liquid stream. More particularly, the presentinvention relates to a thermodynamically efficient purified liquidproduction method and apparatus using an improved purification chamber.

BACKGROUND OF THE INVENTION

[0002] Highly pressurized, purified liquid carbon dioxide is requiredfor a variety of industrial processes. Often carbon dioxide as a bulksource stream is provided for purification as a vapor from a bulk carbondioxide storage tank. For example, U.S. Pat. No. 6,327,872 discloses amethod and apparatus for producing a pressurized high purity liquidcarbon dioxide stream in which a carbon dioxide vapor feed stream ispurified within a purifying filter and then condensed within acondenser. The resulting liquid is then alternately introduced anddispensed from two pressure accumulation chambers, which are heated byelectrical heaters to pressurize the liquid to the desired deliverypressure.

[0003] However, system and size constraints often make it inefficient orimpractical from a cost or logistical standpoint to manufacture purifiedliquid carbon dioxide from a vapor source. Indeed, carbon dioxidepurification facilities using carbon dioxide from a bulk tank as a vaporsource exhibit various complications, which become more problematic forhigh throughput systems. When a carbon dioxide vapor stream is used,there is a substantial heating load on the bulk tank pressure buildingsystem, which increases the likelihood of ice accumulating and blockingthe pressure building system heat exchanger. In addition, such systemsrequire the application of supplemental heating sources to maintainsystem pressure and vaporization. Such bulk tank vapor source systemsalso suffer from impurity buildup that results in significant timeoff-line for costly periodic maintenance and repair.

[0004] Therefore, a need exists for alternative method and apparatus forproducing purified liquid carbon dioxide, or more generally a purifiedliquid, with improved performance, increased energy efficiency andreduced equipment cost.

SUMMARY OF THE INVENTION

[0005] The present invention generally relates to a method and apparatusfor producing a purified liquid. The method and apparatus can be appliedto the production of purified liquid carbon dioxide, nitrous oxide,ammonia and fluorocarbons.

[0006] One aspect of the present invention provides a method forproducing a purified liquid stream. In one embodiment, a methodcomprises providing a feed stream source, introducing under pressure afeed stream from the source to a purification vessel, and supplying heatto the feed stream by heat exchange with a compressed refrigerant vaporstream in a first heat exchanger. The feed stream is distilled to form apurified vapor, which is condensed to form purified liquid by heatexchange with a refrigerant liquid stream in a second heat exchanger.The purified liquid stream is then withdrawn from the purificationvessel. The refrigerant liquid stream and the compressed refrigerantvapor stream are provided in a refrigerant flow network comprising thefirst and second heat exchangers.

[0007] In another embodiment, the method comprises providing a liquidmaterial source and introducing under pressure a liquid feed stream fromthe liquid material source to a purification vessel via a substantiallyfree flow connection, with the purification vessel comprising adistillation column assembly and a collection chamber positionedannularly about the distillation column assembly. The liquid feed streamis purified in the distillation column assembly to produce a purifiedliquid, which is stored in the collection chamber.

[0008] Another embodiment relates to a method comprising providing aliquid material source and introducing, under pressure, a liquid feedstream from the source to a purification vessel via a substantially freeflow connection. The liquid material is selected from the groupconsisting of carbon dioxide, nitrous oxide, ammonia and fluorocarbons.The purification vessel comprises a distillation column assembly and acollection chamber positioned annularly about the distillation columnassembly. The liquid feed stream is vaporized into a vapor; which isdirected through a distillation column in the distillation columnassembly to purify the vapor. The purified vapor is condensed into apurified liquid; which is collected in the collection chamber to apredetermined volume. When the predetermined volume has been exceeded, aportion of the purified liquid is returned from the bottom of thecollection chamber to the distillation column assembly.

[0009] In yet another embodiment, the method comprises providing aliquid material source and introducing, under pressure, a liquid feedstream from the source to a purification vessel via a substantially freeflow connection. The liquid material is selected from the groupconsisting of carbon dioxide, nitrous oxide, ammonia and fluorocarbons.The purification vessel comprises a distillation column assembly and acollection chamber positioned annularly about the distillation columnassembly. The liquid feed stream is vaporized to produce a vapor; whichis directed through a distillation column of the distillation columnassembly to produce a purified vapor. The method further comprisesproviding a condenser refrigeration system comprising at least onecondenser inside the distillation column assembly; condensing thepurified vapor into a purified liquid in the at least one condenser; andcollecting the purified liquid in the collection chamber to apredetermined volume. An amount of the purified liquid is withdrawn fromthe collection chamber and subjected to a pressure of from about 1100 toabout 3000 psia. Heat byproduct from the condenser refrigeration systemis directed to a heat exchanger for heating the purified liquid to apredetermined delivery or storage temperature; and the heat byproduct isgenerated in part by the condensation of the purified vapor in the atleast one condenser.

[0010] Another aspect of the invention relates to an apparatus forproducing a purified liquid stream. In one embodiment, the apparatuscomprises a purification vessel in connection with a bulk materialsource. The purification vessel comprises an intake for admitting a bulkmaterial feed stream from the source, a distillation assembly comprisinga distillation column for forming a purified vapor from the bulkmaterial feed stream and a condenser for condensing the purified vaporinto a purified liquid, and an annular collection chamber surroundingthe distillation column for collecting the purified liquid. Theapparatus also comprises a refrigerant flow network in communicationwith the bulk material feed stream and the purified vapor for providingheat to the bulk feed stream and cooling to condense the purified vaporto the liquid after the purified vapor exits the distillation column.The apparatus can be used for producing purified carbon dioxide, nitrousoxide, ammonia and fluorocarbons.

[0011] Another aspect of the invention provides a purification vesselcomprising: a distillation column assembly having an inlet for admittingan amount of material to be purified and an outlet for releasing anamount of purified material, a heat exchanger in contact with thematerial to be purified, a packed distillation column having a columninlet and column outlet through which material to be purified passes,the exchanger positioned below the column inlet, and a condenser locatedproximate to the column outlet; and an annular chamber surrounding thepacked distillation column, the annular chamber having an inlet forcollecting purified material and an outlet for releasing the collectedpurified material.

[0012] Yet another aspect of the invention provides an annular chamberfor collecting purified liquid carbon dioxide from a distillation columncomprising a substantially cylindrical vertical inner wall, a chamberbottom extending radially outward from the inner wall a predetermineddistance to a substantially cylindrical vertical outer wall, the innerwall having a diameter dimensioned to surround a packed distillationcolumn.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] While the specification concludes with claims distinctly pointingout the subject matter that applicants regard as their invention, it isbelieved that the invention will be better understood when taken inconnection with the following drawings.

[0014]FIG. 1 is a schematic representation of one embodiment of theprocess and apparatus of the present invention.

[0015]FIG. 2 is a schematic representation of one embodiment of theprocess and apparatus of the present invention implemented in anapplication.

[0016] FIGS. 3-5 are exposed perspective views of embodiments of thepurification vessel of the present invention.

[0017]FIG. 6 is a schematic view of another embodiment of thepurification vessel of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] With reference to FIG. 1, an apparatus 1 is illustrated inaccordance with the present invention. A feed stream of liquid carbondioxide, such as from a standard 300 psig bulk tank (not shown inFIG. 1) enters the apparatus 1 via line 10. Line 10 connects to thebottom of purification and storage vessel 12 at port 14. Purificationand storage vessel 12 comprises a distillation column assembly 13surrounded by an annular collection chamber 32 that is preferably anintegral chamber—i.e., as an integral component of the purificationvessel 12. Line 10 is specifically designed to allow the substantiallyfree flow (without significant restriction) of carbon dioxide back andforth from the bulk carbon dioxide supply to the distillation columnassembly 13 of purification and storage vessel 12. According to thepresent invention, it may be desirable to slow down the free flow rateof liquid carbon dioxide into the system 1 to ensure that the levelcontrol system has adequate time to respond to the influx of material.Since at least some degree of check on a pure free flow system may be ineffect at some times during system operation, there is more precisely asubstantially free flow arrangement between the bulk carbon dioxidesource and the system 1. During operation, carbon dioxide enters thebottom of the purification and storage vessel 12 into distillationcolumn assembly 13 and begins to fill the distillation column assembly13 at least to a level 15 that substantially submerges the boilup heatexchanger 16.

[0019] The level of liquid carbon dioxide admitted to the purificationand storage vessel 12 is controlled by adjusting the duty in the heatexchanger 16 to maintain a level setpoint as the liquid carbon dioxide17 during operation is vaporized continuously from a liquid state. Whenthe liquid carbon dioxide level is below setpoint, less duty is suppliedresulting in a reduced carbon dioxide vaporization rate. When the liquidcarbon dioxide level is above setpoint, more duty is supplied causing anincreased carbon dioxide vaporization rate. It is understood that thedistillation column assembly 13 has three regions: a boilup region inthe bottom where liquid carbon dioxide is exposed to heat for thepurpose of vaporizing the liquid; a distillation region located abovethe boilup region comprising a distillation column; and a condensingregion above the column whereby the purified vapor emerging from thecolumn is exposed to a heat exchanger for purposes of condensing thepurified vapor into purified liquid carbon dioxide. The distillationcolumn can generally be either a packed column or any suitable trayedcolumn, although a packed column is used in this discussion forillustrative purpose.

[0020] In one embodiment of the present invention, waste carbon dioxidevapor from an online application is redirected via a carbon dioxidevapor recycle line 18 to the carbon dioxide purification and storagevessel 12 from, for example, an abatement and recovery process. (Notshown in FIG. 1—See 121 in FIG. 2). The recycle line 18 as shown entersthe purification and storage vessel 12 at a point above the level of theliquid carbon dioxide and the recycled carbon dioxide vapor combines orcommingles with the bulk carbon dioxide delivered to the vessel via line10.

[0021] As heat exchanger 16 causes the vaporization of the liquid carbondioxide 17, carbon dioxide vapor flows upwardly though a first packeddistillation column 20 and contacts condensed liquid carbon dioxidemoving downwardly through column 20. Such counter-current liquid-vaporcontact removes heavy impurities from the ascending carbon dioxide vaporand returns the impurities to the liquid carbon dioxide 17 in the bottom22 of vessel 12. Some carbon dioxide vapor emerges from the top of thefirst column 20 and is substantially free of heavy impurities. Carbondioxide liquid 17 located at the bottom 22 of vessel 12 is periodicallyvented via line 78 to prevent an excessive buildup of impurities in thevessel 12.

[0022] As shown in FIG. 1, the majority of the ascending carbon dioxidevapor emerging from the top of the first column 20 flows into the bottomof the second packed distillation column 24. Once in the second column24, the ascending carbon dioxide vapor contacts condensed carbon dioxideliquid descending through the second column 24. This counter-currentliquid-vapor contact concentrates light impurities in the ascendingcarbon dioxide vapor and decreases the concentration of the lightimpurities in the descending liquid carbon dioxide. The carbon dioxidevapor emerging from the top of the second packed column 24 is partiallycondensed by heat exchanger 26 resulting in condensed carbon dioxidebeing returned to the top of second packed column 24 as reflux. The flowrate of the reflux is controlled by setting the duty in the heatexchanger 26. Line 28 extends from the top of the second packed column24 within distillation chamber assembly 13 and contains valve 30, whichis periodically opened to vent accumulated vapor containing light ornon-condensable impurities.

[0023] The liquid reflux entering the top of the second packed column 24passes down through the column and is collected into the annularcollection chamber 32 that surrounds the first packed column 20 indistillation chamber assembly 13. The flow rate of the liquid productcollection is controlled by setting the duty in heat exchanger 26. Heatexchanger 34 is used to generate liquid reflux for the first packedcolumn 20. The liquid reflux thus flows down the first packed column 20to the bottom of distillation chamber 13 and is ready once again forboilup. Heat exchanger 34 provides reflux only for the first packedcolumn 20.

[0024] As described above, liquid carbon dioxide condensed by heatexchanger 26 is collected as product in the annular collection chamber32 that surrounds the first packed column. The annular collection tank32 is provided with an overflow tube 36 that returns liquid carbondioxide from the bottom of the annular tank to the top of the firstpacked column 20 where it serves as an additional liquid reflux. Theoverflow tube 36 ensures that carbon dioxide inventory is held on afirst in, first out basis. In other words, according to one embodimentof the present invention, the oldest carbon dioxide at the bottom of theannular collection chamber 32 is returned to the first packed column 20,and the annular tank 32 is continuously purged by incoming purifiedcarbon dioxide during periods of low carbon dioxide product demand.Alternatively, liquid carbon dioxide condensed by heat exchanger 26could be directed to the bottom of the annular collection chamber 32 andthe collection chamber 32 could be provided with an overflow thatreturns liquid carbon dioxide from the top of the annular chamber 32 tothe top of the packed distillation column 20. This would also returnolder carbon dioxide, now at the top of the annular chamber 32 to thepacked column 20. Purified liquid carbon dioxide product is withdrawnfrom the bottom of the annular collection chamber 32 as needed by thepressurization and delivery system of the apparatus.

[0025] During periods of low product demand, as pressure within thevessel 12 builds up, it is understood and indeed is one advantageousaspect of the present invention that back pressure in the system mayallow a flow of impure liquid carbon dioxide from the bottom of vessel12 to flow through the free flow line 10 back into the bulk liquidcarbon dioxide source due to the substantially free flow nature of line10. For example, when, as a temporary condition, more carbon dioxide isreceived from the recycle vapor line 18 than is withdrawn from theannular collection chamber 32 as product, the excess carbon dioxide iscondensed to liquid and sent to the bulk liquid carbon dioxide sourcevia the free flow line 10 for storage. When the temporary condition ofexcess recycle vapor is over, the condensed liquid placed in the bulkliquid source is withdrawn and processed normally as previouslydescribed. One resulting advantage from this efficient use of recyclevapor returned to the purification vessel is to reduce consumption ofthe bulk source liquid carbon dioxide in the purification system.

[0026] All duty for both cooling and heating is provided by a singleclosed cycle refrigeration system. The refrigerant selected forreference purposes is preferably R22 but may be any suitable refrigerantsuch as, but not limited to R134A depending upon commercial objectives.Substitution of refrigerant may result in different system pressures andtemperatures as appropriate. The refrigerant accumulator 38 is sized tocontain the entire inventory of refrigerant as liquid. The accumulator38 separates liquid refrigerant from the mixed phase feed streams andensures that saturated liquid refrigerant is available as feed to thecondenser heat exchangers 26, 34. Pressure is controlled by ventingrefrigerant vapor through valve 40 to the refrigerant compressor 54. Thetwo condenser heat exchangers 26, 34 both take liquid refrigerant fromthe refrigerant accumulator 38 through a free flowing connection line42, 44 respectively. Liquid refrigerant is intended to flow freelythrough lines 42, 44 without restriction. Liquid refrigerant enters thebottom of each condenser 26, 34, is vaporized and exits the top of theheat exchangers as a vapor. The duty is controlled by controlling theflow rate of vapor refrigerant leaving the top of each heat exchanger.The level of liquid refrigerant inside each heat exchanger isself-adjusting so long as the maximum heat exchange capacity is notexceeded.

[0027] Flow control valves 46, 48 are used to control the vapor flowrate and therefore the duty of heat exchangers 26, 34 respectively.After leaving the valves, the refrigerant vapor flows on to therefrigerant compressor suction manifold 47. A liquid trap 50 in thesuction manifold collects any liquid refrigerant that may be present andprevents it from entering the compressor where it could cause damage.The collected liquid is slowly vaporized either by electric heaters 52or other means as appropriate. Refrigerant vapor from the suctionmanifold is compressed by a compressor 54 and discharged as a hot highpressure gas. A first portion of the compressor discharge refrigerantvapor is sent to boilup heat exchanger 16 where it condenses to liquidcausing boilup of the liquid carbon dioxide in the bottom ofdistillation chamber 13. High pressure liquid refrigerant accumulatesinside heat exchanger 16 and the duty is controlled by controlling theflow rate of liquid refrigerant leaving the heat exchanger 16 by flowcontrol valve 56. When more duty is required to reduce the carbondioxide level within the annular collection chamber 32, valve 56 isopened further allowing more liquid refrigerant to leave heat exchanger16 and consequently more vapor refrigerant to enter and condense andsupply additional duty. When less duty is required, valve 56 is closed,further causing a reduction in the flow rate of refrigerant vapor and aconsequent decrease in duty. The liquid refrigerant passing throughvalve 56 is sent to the refrigerant accumulator 38 via line 58.

[0028] As an alternative, duty in heat exchanger 16 can be controlled byusing valve 56 and pressure transducer 60 as a back pressure control andvalve 62 to control refrigerant flow rate. The back pressure setpoint ishigh enough that the dew point of R22 vapor inside heat exchanger 16provides enough temperature differential to drive heat transfer againstthe boiling liquid carbon dioxide. For example, a pressure setpoint of50 psia would give a R22 dew point of −11C and provide a temperaturedifferential of 8.8C against boiling carbon dioxide at 280 psia and−19.8C. Duty in heat exchanger 16 is controlled by controlling flow rateof R22 vapor through valve 62 as required to maintain the liquid carbondioxide level setpoint. This approach may allow the duty in heatexchanger 16 to be changed more quickly because the liquid R22 inventorywithin heat exchanger 16 can be removed or added more quickly.

[0029] A second portion of the compressor discharge refrigerant vapor issent through pressure regulator 64 and then on to the product warmingheat exchanger 66. The function of regulator 64 is to reduce thepressure so that the dew point of the resulting reduced pressurerefrigerant vapor is close to the desired delivery temperature of thehigh pressure purified liquid carbon dioxide product. The reducedpressure refrigerant vapor is fed beneath a pool of liquid refrigerantin heat exchanger 66 to remove any superheat. The resultingde-superheated vapor condenses at the dew point established by regulator64 and warms the high-pressure purified liquid carbon dioxide product toa temperature approximating the dew point. As shown in FIG. 1, the levelof condensed liquid refrigerant is controlled by a float operated levelcontrol valve 68 and sent to the refrigerant accumulator 38.

[0030] When needed, a portion of the compressor discharge refrigerantvapor is returned to the compressor suction to prevent the suctionpressure from falling below desired or required operatingspecifications. A hot gas bypass system consisting of pressure regulator70 senses pressure in the compressor suction inlet 49 and opens toreturn refrigerant vapor from the compressor discharge 51 to maintainthe compressor suction pressure within specifications.

[0031] The remaining balance of the compressor discharge refrigerantvapor is sent to an air-cooled condenser 72 where it is condensed.Refrigerant flow rate is controlled by a float-operated valve 73 and theliquid refrigerant is sent to the refrigerant accumulator 38.

[0032] To prevent the compressor suction from exceeding temperaturespecifications, a temperature control valve 53 opens to supply acontrolled flow rate of liquid refrigerant from the condenser 72 to thecompressor suction. The liquid refrigerant flashes to vapor and coolsthe compressor suction. Suction cooling is required only for extendedperiods of operation with hot-gas bypass flow rates.

[0033] Purified liquid carbon dioxide leaves the annular collectionchamber 32 via line 37 and is first sub-cooled in heat exchanger 74 andthen pumped to nominal pressure of from about 1100 to about 3000 psigpressure by pump 76. To ensure NPSH requirements of the pump 76 are met,both the incoming liquid carbon dioxide and the pump itself are cooledbelow the boiling point of the purified liquid carbon dioxide. Therefrigeration to accomplish this cooling is provided by flashing liquidcarbon dioxide taken from the bottom of vessel 12 in the distillationchamber 13 as shown via line 78. This liquid carbon dioxide contains anincreased concentration of heavy impurities and must be vented from thesystem regularly. According to the present invention, however, at leastpart of the refrigeration energy is recovered from this waste stream byheat exchange with the pump 76 and liquid carbon dioxide from line 37.As shown in FIG. 1, after passing through flow control valve 80, thecarbon dioxide acting as refrigerant passes through the pump jacket heatexchanger 82 and pump feed heat exchanger 74 where it is vaporized. Theresulting vapor passes through back pressure regulator 84 which sets theflash pressure and then flows out the heavies vent 86 where it isdischarged to the atmosphere. This efficient use of the waste streamcannot readily be achieved in other purification systems, e.g., one inwhich a vapor carbon dioxide feed stream is supplied from a bulk tank.In those systems, accumulation of heavy impurities in the liquid carbondioxide supply will eventually require the entire liquid content to bediscarded. Provision is also made to separately vent liquid via valve 87from the bottom of vessel 12 if required for control of heavy impurityconcentration in vessel 12.

[0034] The high pressure, but still cold purified liquid carbon dioxidewhich leaves pump 76 is warmed to ambient temperature in heat exchanger66 to prevent possible condensation of atmospheric moisture on the linescarrying high pressure purified liquid carbon dioxide product out of theinventive system and apparatus 1 via line 88. A back pressure regulator90 ensures that the carbon dioxide pump 76 is not damaged should theflow of high pressure carbon dioxide product become blocked.

[0035] It is understood that the purification and storage vessel will bemade from materials able to withstand the processing regimens andrequirements of the system which is understood to be a low temperaturesystem. For example, the annular storage chamber and distillation columnassembly are preferably made from 304, 316 and 316L stainless steel,with 304 stainless steel being most preferred. Furthermore, theapparatus and method disclosed above can also be used with othersuitable liquids, such as nitrous oxide, ammonia and fluorocarbons.

[0036]FIG. 2 shows the carbon dioxide supply system 1 of FIG. 1incorporated into a desired point of use application 100. In thisillustration, high pressure carbon dioxide liquid from system 1 isdelivered for use in a process tool 102. A waste stream containingcarbon dioxide from the process tool is then treated and recycled backto system 1 for purification and re-use. Line 10 directs the bulk liquidcarbon dioxide from a bulk carbon dioxide source 92 to the purificationvessel 12. The purification vessel is in communication with a lowpressure liquid accumulator 32 that is preferably an annular tankcontained within the purification and storage vessel 12. A high pressureliquid pump 76 establishes and maintains a delivery of the pure carbondioxide to equipment 106, e.g., at a pressure of at least about 1100psig, preferably from about 1100 to about 3500 psig, and more preferablybetween about 3000 and about 3500 psig, and a temperature of about 20Cto about 40C. Depending on the specific application, however, otherdelivery pressures may also be used.

[0037] In the illustrated embodiment, high pressure purified liquidcarbon dioxide is directed via line 98 of system 1 to a pure carbondioxide accumulator 106 (high pressure liquid). Depending on theapplication, the purified liquid carbon dioxide may also be directed toother equipment 104 and/or 108 (e.g., mixer for mixing carbon dioxidewith other fluids, or temperature and pressure controller) prior tobeing supplied to the tool 102. Used carbon dioxide is vented from thetool environment 102 along with impurities via line 112 for processingthrough liquid/vapor separator 115 and various waste treatment stages,which may include, for example, vapor scrubbing 114, chemical abatement118, and waste packaging and storage 120. Cleansed carbon dioxide vaporis then purged to the atmosphere via line 122 or directed via line 121(low pressure vapor) to carbon dioxide vapor recycle line 18 of system1.

[0038] As an example, the application 100 may be a processing step insemiconductor fabrication that requires the use of high pressure, orsupercritical, purified carbon dioxide, e.g., wafer drying, resiststripping, etch residue removal, among others. In this case, tool 102 isany suitable processing tool such as a dryer, resist stripper or cleanerlocated inside a clean room (or the “fab” area), while support equipment104, 106, 108 and waste treatment equipment 114, 115, 118 and 120 aretypically located in the “sub-fab” area, with supply and return linescoupling these equipment to the carbon dioxide supply system 1 locatedoutdoors. It is also understood that controllers and sensors areprovided in many of the equipment in such an application in order toallow proper process monitoring, control and automation.

[0039] As stated above, in one embodiment of the present invention,establishing a substantially free flowing connection between the bulkcarbon dioxide source and the purifying vessel allows the liquid carbondioxide to move back and forth as needed between the bulk carbon dioxidestorage tank source and the purifying vessel, which in the preferredembodiment of the present invention is a multi-purpose vessel. Thisensures that the pressures are substantially equal between the sourcetank and the vessel. As carbon dioxide is removed from the vessel, suchas to satisfy product demand, the pressure within the vessel decreasesand liquid carbon dioxide flows from the bulk storage tank to re-fillthe vessel. The purification vessel of the present invention as shown inFIGS. 1 and 3-6 comprise two packed columns within a first chamber toeffect distillation and purification of the liquid carbon dioxide, alongwith an annular storage chamber positioned to surround the firstdistillation column.

[0040] FIGS. 3-5 show an exposed perspective view of the purificationsystem. In one embodiment of the present invention, as shown in FIG. 3,the purification vessel 12 comprises two major functions or systems—thatof purification and storage of the purified liquid. Unlike most knowncarbon dioxide purification systems, which rely on the use of filter ora single condensation step, the present invention uses distillation forpurification, and in particular, multistage distillation with at leastone trayed or packed column. The distillation column assembly 13comprises the area for boilup 11 at the bottom 22 of vessel 12, packeddistillation columns 20, 24 and condensing area 15 located at the top 23of the purification vessel 12. Purified carbon dioxide vapor condensesand descends within the purification vessel 12 and collects within aregion of the purification vessel 12 forming the annular collectionchamber 32. In this way, the purification vessel also stores thepurified liquid carbon dioxide until it is ready to be directed from theannular chamber 32 via outlet 37 to the point-of use of other usedemand. This contrasts with other purification systems, e.g., filterpurifiers, where separate purification and storage vessels are required.

[0041] In a further embodiment, as shown in FIG. 4, the annularcollection chamber 32 is shown as formed by a separate substantiallycylindrical piece 12 c that is welded via weld seals 31 to the upper 12a and lower 12 b sections of the purification vessel. In thisembodiment, the inner cylindrical wall 35 of the piece 12 c has adiameter about equal to or just slightly larger than the diameter of theouter wall 23 of the distillation column 20. As shown, the floor 29 ofpiece 12 c is proximate to flange 31 of bottom section 12 b. Therefore,in this embodiment the annular storage chamber 32 is created by aseparate structure 12 c that is assembled with the other structuralcomponents to make up the overall purification vessel 12.

[0042] In a still further embodiment, FIG. 5 shows the annularcollection chamber 32 as formed by a discrete annular storage vessel 39dimensioned to fit within purification vessel 12. In this embodiment,the annular storage vessel 39 has a substantially cylindrical inner wall35 with a diameter dimensioned to be equal to or slightly exceed thediameter of the outer wall of the distillation column 20. In addition,the diameter of the substantially cylindrical outer wall of the annularstorage vessel 39 is slightly less than the diameter of the inner wall33 of the purification vessel. Therefore in this embodiment, the annularstorage chamber 32 is created by a separate structure 39 that acts as aninset piece onto the distillation column for the collection of thepurified liquid carbon dioxide product.

[0043]FIG. 6 is a schematic representation of another purificationvessel of the present invention. In this embodiment, the vessel 12comprises a distillation column assembly that comprises a heat exchanger16, first distillation column 20, second distillation column 24 andcondenser 26. Annular collection chamber 32 is shown encasing the firstdistillation column 20. Carbon dioxide liquid from condenser 26 isprovided as reflux to the top of distillation column 24. Purified liquidcarbon dioxide collected at a reservoir 6 at the bottom of column 24 isdirected to the annular chamber 32 via duct 7. In this embodiment,liquid carbon dioxide from reservoir 6 is also provided as reflux todistillation column 20 via duct 8 and control valve 9.

[0044] Embodiments of the present invention provide various features andadvantages, some of which are highlighted below. For example, enhancedconservation of thermal energy is achieved by coupling the productcondenser 26 and boil-up heat exchanger 16 to allow efficient use ofthermal energy within the refrigerant flow network. Liquid carbondioxide in the bottom of the vessel is vaporized in a boil-up heatexchanger 16 by heat transfer with a condensing refrigerant vapor. Therefrigerant vapor, which is a portion of a compressed refrigerant vaporstream in a refrigerant flow network (used for refrigeration duty inother equipment or elsewhere in the process), enters heat exchanger 16via inlet 63. In general, a large portion of the heat contained in therefrigerant vapor originates from the condensing duty at condenser 26.Heat transfer between the refrigerant vapor and the liquid carbondioxide causes liquid carbon dioxide to vaporize and the refrigerantvapor to condense. The condensed refrigerant exits the heat exchanger 16via line 58 and returns to the refrigerant flow network. Thisarrangement allows the otherwise wasted heat from the refrigerant vapor,also referred to as heat byproduct, to be used for vaporization ofliquid carbon dioxide. By contrast, in known processes where vaporcarbon dioxide is drawn from the bulk source and a refrigerant stream isused to condense the vapor carbon dioxide, all the heat resulting fromthe condensing refrigerant vapor is rejected as waste heat to theatmosphere. Additional energy must often be supplied, such as byadditional heaters, to meet heating needs in other parts of the systemor process, e.g., for vaporization of the carbon dioxide feed stream.

[0045] Still further, in the present invention, the flow rate of thecondensing refrigerant is controlled by using a control valve (shown asvalve 56 in line 58 of FIG. 1) to adjust the flow rate of liquidrefrigerant leaving the boil-up heat exchanger 16 based on the level ofliquid carbon dioxide in the bottom of the purification vessel 12. Asshown in FIG. 1, the level of liquid carbon dioxide in the purificationvessel 12 is monitored by a level indicating controller (LIC), whichalso provides a control signal to control valve 56. When the liquidlevel is above a predetermined volume or set point, the control valve 56opens, further increasing the refrigerant flow rate through heatexchanger 16 and increasing the boil-up rate of the carbon dioxidecausing the level to decrease. When the liquid level is below set point,the refrigerant flow rate is decreased causing the carbon dioxide levelto increase. In addition, according to the present invention, theresulting liquid refrigerant is directed to a liquid refrigerant storagevessel to be used elsewhere in the purification process.

[0046] Furthermore, the liquid refrigerant is withdrawn from liquidrefrigerant storage 38 to operate both the reflux and product condensers(or heat exchangers) 34 and 26. One preferred way to control bothcondensers is to regulate the flow rate of refrigerant vapor leavingeach condenser using, for example, flow control valves 46 and 48, andallow free access (free flow) between the condenser and the liquidrefrigerant storage vessel for liquid refrigerant to both enter and exitthe condenser. With this free flow design, the level of refrigerantinsider the condensers are self-adjusting, and improved control of thecondenser duty can be achieved.

[0047] Yet another feature of the present apparatus—the overflow tube 36in the collection chamber 32, also contributes to performanceenhancement. When the annular collection chamber 32 (surrounding thedistillation columns) becomes full, liquid carbon dioxide is removed viaoverflow tube 36 from the bottom of the chamber 32. Thus, freshlypurified liquid carbon dioxide is used to displace the oldest carbondioxide, which is transferred from the bottom of the annular collectionchamber 32 to the top of the distillation column 20 where it isre-purified. When the system demand for product is less than capacity,the unneeded or extra capacity is used to re-purify the previouslyaccumulated purified liquid carbon dioxide product. The increased liquidreflux to the distillation column 20 also results in a higher purityproduct. With this arrangement of the overflow tube 36, therefrigeration system continues to run as before during periods of lowdemand without requiring adjustment, and less liquid carbon dioxide iswithdrawn from the bulk source carbon dioxide. By contrast, in theabsence of the overflow tube 36, adjustment of the refrigeration systemwould be required in order to turn down the duty of the condenser 26.

[0048] While the present invention has been described with reference topreferred embodiments, as will occur to those skilled in the art,numerous additions, changes, and omissions can be made without departingfrom the spirit and scope of the present invention. For example, whiletwo distillation columns are used in the above illustrations,embodiments of present invention can also be practiced in a system witha single distillation column. Furthermore, although it is advantageousto provide to the purification vessel a feed stream comprising primarilyof liquid, certain aspects or embodiments of the present invention canbe practiced generally with any fluid feed stream, including a vaporfeed stream. In addition, the method and apparatus disclosed herein cangenerally be applied or adapted to produce other purified liquidmaterials, such as ammonia, nitrous oxide, or fluorocarbons, etc. Highpurity nitrous oxide, ammonia and fluorocarbons also have potentialapplications in semiconductor fabrication. It is further understood thatembodiments of the invention may be practiced with differentcombinations of one or more features disclosed herein. Thus, althoughone aspect of the invention provides a purification vessel with aninternal storage chamber, an external storage chamber may also beadvantageously used in conjunction with other features of the invention.

We claim:
 1. A method for producing a purified liquid stream comprising:providing a feed stream source; introducing under pressure a feed streamfrom the source to a purification vessel; supplying heat to the feedstream by heat exchange with a compressed refrigerant vapor stream in afirst heat exchanger; distilling the feed stream to form a purifiedvapor; condensing the purified vapor to form purified liquid by heatexchange with a refrigerant liquid stream in a second heat exchanger;wherein the refrigerant liquid stream and the compressed refrigerantvapor stream are provided in a refrigerant flow network comprising thefirst and second heat exchangers; and withdrawing the purified liquidstream from the purification vessel.
 2. The method of claim 1, whereinthe feed stream is a liquid stream, and a substantially free flowconnection is provided between the source and the purification vessel.3. The method of claim 2, wherein the feed stream is selected from thegroup consisting of carbon dioxide, nitrous oxide, ammonia andfluorocarbons.
 4. The method of claim 1, wherein the feed stream is aliquid carbon dioxide stream, and at least a portion of the heatsupplied to the liquid carbon dioxide feed stream at the first heatexchanger is generated from the condensation of the purified carbondioxide vapor at the second heat exchanger.
 5. A method for producing apurified liquid stream comprising: providing a liquid material source;introducing under pressure a liquid feed stream from the liquid materialsource to a purification vessel via a substantially free flowconnection, the purification vessel comprising a distillation columnassembly and a collection chamber positioned annularly about thedistillation column assembly; purifying the liquid feed stream in thedistillation column assembly to produce a purified liquid; and storingthe purified liquid in the collection chamber.
 6. The method of claim 5,wherein the liquid material is selected from the group consisting ofcarbon dioxide, nitrous oxide, ammonia and fluorocarbons.
 7. The methodof claim 6, further comprising the step of: directing a recycle vapor ofthe material having impurities via a line to the distillation columnassembly in the purification vessel.
 8. The method of claim 5, furthercomprising the step of: directing the liquid from the bottom of thepurification vessel through a line into the liquid material source. 9.The method of claim 5, further comprising the step of: withdrawing thepurified liquid stream from the collection chamber of the purificationvessel.
 10. A method for producing a purified liquid stream comprising:providing a liquid material source wherein the liquid material isselected from the group consisting of carbon dioxide, nitrous oxide,ammonia and fluorocarbons; introducing under pressure a liquid feedstream from the liquid material source to a purification vessel via asubstantially free flow connection, the purification vessel comprising adistillation column assembly and a collection chamber positionedannularly about the distillation column assembly; vaporizing the liquidfeed stream into a vapor; directing the vapor through a distillationcolumn in the distillation column assembly to purify the vapor;condensing the purified vapor into a purified liquid; collecting thepurified liquid in the collection chamber to a predetermined volume; andreturning a portion of the purified liquid from the bottom of thecollection chamber to the distillation column assembly when thepredetermined volume has been exceeded.
 11. A method for producing apurified liquid stream comprising: providing a liquid material sourcewherein the liquid material is selected from the group consisting ofcarbon dioxide, nitrous oxide, ammonia and fluorocarbons; introducingunder pressure a liquid feed stream from the liquid material source to apurification vessel via a substantially free flow connection, thepurification vessel comprising a distillation column assembly and acollection chamber positioned annularly about the distillation columnassembly; vaporizing the liquid feed stream to produce a vapor;directing the vapor through a distillation column of the distillationcolumn assembly to produce a purified vapor; providing a condenserrefrigeration system comprising at least one condenser inside thedistillation column assembly; condensing the purified vapor into apurified liquid in the at least one condenser; collecting the purifiedliquid in the collection chamber to a predetermined volume; withdrawingan amount of the purified liquid from the collection chamber andsubjecting the purified liquid to a pressure of from about 1100 to about3000 psia; and directing heat byproduct from the condenser refrigerationsystem to a heat exchanger for heating the purified liquid to apredetermined delivery or storage temperature; wherein the heatbyproduct is generated in part by the condensation of the purified vaporin the at least one condenser.
 12. An apparatus for producing a purifiedliquid stream comprising: a purification vessel in connection with abulk material source, the vessel comprising an intake for admitting abulk material feed stream from the source, a distillation assemblycomprising a distillation column for forming a purified vapor from thebulk material feed stream and a condenser for condensing the purifiedvapor into a purified liquid, and an annular collection chambersurrounding the distillation column for collecting the purified liquid;and a refrigerant flow network in communication with the bulk materialfeed stream and the purified vapor for providing heat to the bulkmaterial feed stream and cooling to condense the purified vapor into theliquid after the purified vapor exits the distillation column.
 13. Theapparatus of claim 12, wherein the bulk material is selected from thegroup consisting of carbon dioxide, nitrous oxide, ammonia andfluorocarbons.
 14. The apparatus of claim 12, wherein the intake of thevessel is a substantially free flow intake and the bulk material feedstream is a liquid stream.
 15. The apparatus of claim 14, furthercomprising means for transferring a portion of the purified liquid fromthe annular collection chamber to the distillation column for reflux.16. The apparatus of claim 14, further comprising means for transferringa portion of the purified liquid formed from the condenser to theannular collection chamber and another portion of the purified liquid tothe top of the distillation column.
 17. The apparatus of claim 14,wherein a heat byproduct is produced from the refrigerant flow network,the heat byproduct directed to warm the purified liquid.
 18. Theapparatus of claim 14, wherein the annular collection chamber is anintegral component of the purification vessel.
 19. The apparatus ofclaim 14, wherein the annular collection chamber is a discrete componentof the purification vessel.
 20. The apparatus of claim 14, furthercomprising a recycle feed line for directing a vapor of the materialcontaining impurities from a point-of-use application to thedistillation assembly.
 21. The apparatus of claim 14, further comprisinga recycle feed line for directing a recycle vapor stream of the materialcontaining impurities from a purification treatment to the distillationassembly.
 22. The apparatus of claim 21, wherein the recycle vaporstream from the recycle feed line commingles in the purification vesselwith the bulk liquid material.
 23. The apparatus of claim 22, whereinthe commingled recycle stream and bulk liquid material returns to thebulk material source via the substantially free flow connection.
 24. Apurification vessel comprising: a distillation column assembly having aninlet for admitting an amount of material to be purified and an outletfor releasing an amount of purified material, a heat exchanger incontact with the material to be purified, a packed distillation columnhaving a column inlet and column outlet through which material to bepurified passes, said exchanger positioned below the column inlet, and acondenser located proximate to the column outlet; and an annular chambersurrounding the packed distillation column, the annular chamber havingan inlet for collecting purified material and an outlet for releasingthe collected purified material.
 25. The purification vessel of claim24, wherein the material to be purified is a bulk liquid selected fromcarbon dioxide, nitrous oxide, ammonia and fluorocarbons.
 26. Thepurification vessel of claim 24, wherein the annular collection chamberis integral with the purification vessel.
 27. The purification vessel ofclaim 24, wherein the annular chamber further acts as a storage chamberfor the purified material.
 28. The purification vessel of claim 24,wherein the annular chamber is a discrete component in the purificationvessel.
 29. The purification vessel of claim 24, wherein the annularchamber comprises a common outer wall with the purification vessel. 30.An annular chamber for collecting purified liquid carbon dioxide from adistillation column comprising: a substantially cylindrical verticalinner wall, a chamber bottom extending radially outward from the innerwall a predetermined distance to a substantially cylindrical verticalouter wall, the inner wall having a diameter dimensioned to surround apacked distillation column.
 31. The annular chamber of claim 30, whereinthe annular chamber further acts as a storage chamber for the purifiedliquid carbon dioxide.
 32. The annular chamber of claim 30, wherein thechamber is an open top vessel.
 33. The annular chamber of claim 30,wherein the chamber is a discrete component in a purification vesselcontaining the distillation column.